In the conventional technique for the fabrication of semiconductor photomasks, a glass plate, after being thoroughly cleaned, accepts a relatively thin layer, typically 600 to 800A, of a metal film vapor-deposited or sputtered on one of its surfaces. A layer of photoresist material is then spun on to the metallic layer, and is subsequently exposed to UV light through a master emulsion mask having openings corresponding to those areas where it is desired to subsequently generate semiconductor junctions, etc. After exposure of the photoresist material through the emulsion mask, the mask is removed and the layer of photoresist is developed and processed by means of a suitable solvent, exposing select areas of the underlying metallic layer. A wet acid-based dip is then used to etch the metallic film from the surface of the glass plate in the exposed areas, the remaining photoresist material serving as an etch-mask for the surface covered by it. Following the wet etching process, a water rinse and a drying step are implemented. The remainder of the photoresist material is subsequently removed via an organic solvent dip, followed by an inorganic acid dip required for the removal of inorganic residues commonly embedded in resist material. The photoresist material along with its associated inorganic contaminants can also be removed by a plasma process utilizing the halocarbon-oxygen gaseous mixtures disclosed by the present inventor in his U.S. patent application Ser. No. 173,537, filed Aug. 20, 1971. Following a further water rinse and a drying cycle, the semiconductor photomasks are packaged and stored away from any excessive moisture and destructive dust particles.
Among the problems and drawbacks associated with the metal wet-etching step used in the above-mentioned technique are:
1. Physical degradation of the photoresist etch mask caused by lifting. PA1 2. Enhanced undercutting effects creating undesirable slops of the etched channel. PA1 3. Impairment of line-line resolution due to (1) and/or (2). PA1 4. Severe and rapid chemical degradation of the metalized layer imposing stringent process control requirements. PA1 5. Requirement for post-etch water rinse and drying steps, invariably increasing handling procedures and correspondingly reducing production yields. PA1 6. Short shelf-life of etching solution due to inevitable contamination. PA1 7. Generally very hazardous to personnel and undesirably polluting.
Accordingly, the general object of the present invention is to provide an improved process and new material that overcome the aforementioned problems and provide uniform etching reactions at a moderate and controllable rate.